1. Field of the Invention
The present invention relates to an injection control system of an internal combustion engine for performing a learning operation of an injection quantity.
2. Description of Related Art
A certain controlling method (an injection quantity learning operation) known as a method of controlling an injection quantity of a gasoline engine or a diesel engine estimates the injection quantity (or torque generated by injection) based on a fluctuation of an engine rotation speed, which is caused by combusting the injected fuel, to correct the injection quantity.
A publicly known calculating method disclosed in U.S. Pat. No. 4,667,634 or Unexamined Japanese Patent Application Publication No. H07-59911 calculates the fluctuation of the engine rotation speed (a rotation speed fluctuation δ) by comparing a rotation speed ωt at a top dead center (TDC), which is sensed at a time point t10 in FIG. 6, with a rotation speed ωc at a crank angle of 90° after the TDC (ATDC 90° CA), which is sensed at a time point t11, as shown by a solid line “f” in FIG. 6. Alternatively, the rotation speed fluctuation δ is calculated by comparing the rotation speed ωc at the ATDC 90° CA with a predetermined value. Engine rotation speeds ωa, ωb, ωc, ωd are respectively measured at time points t3, t8, t11, t14, where the crank angle is ATDC 90°. For instance, the rotation speed ωa at the time point t3 is calculated from a period S1 from a time point t2 to a time point t4. In FIG. 6, a period “A” corresponds to an intake stroke of a first cylinder and a compression stroke of a second cylinder. A period “B” corresponds to a compression stroke of the first cylinder. A period “C” corresponds to an expansion stroke of the first cylinder and a compression stroke of a third cylinder. A period “D” corresponds to an exhaustion stroke of the first cylinder and a compression stroke of a fourth cylinder. In FIG. 6, a solid line “a” or a broken line “a′” represents a cylinder pressure P1 of the first cylinder, a solid line “b” represents torque Ti generated by performing a single injection, a solid line “c” represents torque Tc generated by a compression stroke in a next cylinder, in which the injection is performed next, a solid line “d” represents a fluctuation δi of the engine rotation speed ω caused by the single injection on the basis of the engine rotation speed ω0 at a time point t1, a solid line “e” represents a fluctuation δc of the engine rotation speed ω caused by the compression stroke in the next cylinder on the basis of the engine rotation speed ω0 at the time point t1, and a solid line “f” or a broken line “f′” represents the engine rotation speed ω.
The injected fuel is combusted to generate heat, and the heat increases the cylinder pressure. Thus, the crankshaft is rotated through a piston and a connecting rod. Therefore, it can be estimated that the torque generated by the fuel injection is continuously applied to the crankshaft until the increased cylinder pressure decreases to a level provided in the case where the injection is not performed.
If the single injection is performed, the cylinder pressure P1 of the first cylinder is increased from the pressure shown by the broken line “a′” to the pressure shown by the solid line “a” in FIG. 6. The injected fuel is ignited at the time point t10 and an exhaust valve opens at a time point t12. If the rotation speed ω is measured at the ATDC 90° CA (for instance, at the time point t11), the rotation speed ω is measured before the torque corresponding to a partial pressure shown by an area Sp2 in FIG. 6 out of the increase in the cylinder pressure shown by areas Sp1, Sp2 contributes to the increase of the rotation speed ω.
Therefore, if the rotation speed ωc measured at the ATDC 90° CA is compared with the rotation speed ωt measured at the TDC, the rotation speed fluctuation δi caused by the injection cannot be measured accurately. It is because all of the energy generated by combusting the injected fuel has not yet contribute to the rotation of the crankshaft. As a result, there is a problem that the quantity of the actually injected fuel (or the torque Ti generated by the injection) cannot be estimated accurately.
Moreover, the rotation speed fluctuation δ measured by the rotation speed sensor is affected by the compression in the next cylinder, in which the injection is performed next. Therefore, only the value provided by subtracting the rotation speed fluctuation δc caused by the compression in the next cylinder from the rotation speed fluctuation δi caused by the injection can be measured. Actually, the difference δa between the rotation speed ωc and the rotation speed ωt corresponds to a value provided by subtracting the rotation speed fluctuation δam caused by the compression in the next cylinder from the rotation speed fluctuation δap caused by the injection. Therefore, even if the same injection is performed (or even if the rotation speed fluctuation δap caused by the injection is the same), the variations in the rotation speed fluctuation δam caused by the compression in the next cylinder affect the rotation speed fluctuation δa to be measured. As a result, learning accuracy of the injection quantity will be deteriorated.
Even if the rotation speed fluctuation δam caused by the compression in the next cylinder is added to the difference δa between the rotation speed ωc and the rotation speed ωt, the rotation speed fluctuation δap corresponding to the rotation speed ω on the rise due to the injection is measured. As a result, the rotation speed fluctuation δi caused by the injection cannot be measured accurately.